8255(DOCS) Optical Printer Control System 8255(DOCS)
NAME
8255 - how to control 8255 based digital I/O cards
8255 PORTS
The 8255 family of chips (82C55, etc) are usually one chip
solutions to getting 24 bits of programmable digital I/O.
Typically, the chip is configured at a base I/O address,
such as 0310.
There are four ports (base+0, base+1, base+2 and base+3)
that are used to control the chip:
base+0 - PORT #A
base+1 - PORT #B
base+2 - PORT #C
base+3 - CONTROL REGISTER
8255 PROGRAMMING
The "control register" controls the I/O direction of the 3 ports,
and breaks out as follows:
Bit Description
--- --------------------------------------------------
0 Port C (low 4 bits): 1=input, 0=output
1 Port B (all 8 bits): 1=input, 0=output
2 Mode selection: 0=MODE#0, 1=MODE#1
3 Port C (hi 4 bits): 1=input, 0=output
4 Port A (all 8 bits): 1=input, 0=output
_
5 |_ 00 = MODE#0 (basic I/O)
6 _| 01 = MODE#1 (strobed I/O)
1x = MODE#2 (bidirectional bus)
7 Mode set flag (1=active, 0=normal)
No initialization is required to achieve 24 bits of input, which
is the default; during reset, all ports are programmed to be inputs.
The control register must be programmed before doing any I/O
with the three ports A,B and C. Example values for the
Control Register:
0x80 - A,B,C outputs
0x9b - A,B,C inputs
0x92 - A+B input, C=output
8255 PROGRAMMING EXAMPLES
Here's a C programming example that shows how to setup the 8255
such that A+B are inputs, and C is outputs:
/* INITIALIZATION */
base = 0x310;
out(base+3), 0x92; /* A+B=in, C=out */
/* READ/WRITE */
if ( inp(base+0) & 0x01 ) /* read A */
printf("Bit #1 set on port A\n");
else
printf("Bit #1 clear on port A\n");
/* SET PORT C, BIT #1 */
out(base+2, inp(base+2) | 0x01 ); /* write C */
This example shows similar 8255 programming example
in the OPCS system's HOMEDEFS.HOM file, used by the HOME program:
# homedefs.hom
start init_8255
{
# INITIALIZATION
outport 0313 92 # A+B=in, C=out
# READ/WRITE
portset? 0310 01 # test if bit 1 set on Port A
{
print "Bit #1 set on Port A"
}
portclr? 0310 01 # test if bit 1 clear on Port A
{
print "Bit #1 clear on Port A"
}
# SET PORT C, BIT #1
setbit 0312 01
}
end init_8255
8255 CHIP PINOUT
The 82C55A is most commonly found in a 40 pin DIP.
82C55A
Top View
_____ _____
| \/ |
PA3 [| 1 40 |] PA4
PA2 [| 2 39 |] PA5
PA1 [| 3 38 |] PA6
PA0 [| 4 37 |] PA7
RD [| 5 36 |] WR
CS [| 6 35 |] RESET
GND [| 7 34 |] D0
A1 [| 8 33 |] D1
A0 [| 9 32 |] D2
PC7 [| 10 31 |] D3
PC6 [| 11 30 |] D4
PC5 [| 12 29 |] D5
PC4 [| 13 28 |] D6
PC0 [| 14 27 |] D7
PC1 [| 15 26 |] VCC
PC2 [| 16 25 |] PB7
PC3 [| 17 24 |] PB6
PB0 [| 18 23 |] PB5
PB1 [| 19 22 |] PB4
PB2 [| 20 21 |] PB3
|____________|
8255 PORT MONITOR PROGRAM
The OPCS software comes with 8255.exe which can monitor the
real time status of the 8255's I/O ports.
Run '8255.exe' with the hex base address as the first argument:
C:\OPCS\WORK> 8255.EXE 0310
-------- ----
| |
| Base address argument
|
Runs the 8255 program, usually in
the \OPCS\BIN directory.
This tool can be downloaded from https://seriss.com/opcs/ftp/
and the source code on githib at https://github.com/erco77/8255-dos/
CONTROL REGISTER: BASE+3
------------------------
The control register is a single 8 bit port whose byte defines the
I/O direction of all three 8 bit I/O ports A,B,C, and the mode for
that I/O, modes 0,1 and 2 are supported by the 8255 chips.
The control register for each 8255 should be programmed on boot
or OPCS startup to define whether ports A, B and C are inputs
or outputs.
Initializing the control register can be done using the "home"
program, creating a special entry in the HOMEDEFS.HOM file
to initialize the port (see example below), and then invoking
"home 8255_init" during the AUTOEXEC.BAT or via the OPCSDEFS.OPC
setup file that's run when OPCS starts.
Example 8255 init using HOMEDEFS.HOM, where the base address
for the 8255 is 0200:
1. Create a new entry called 'init_8255' in the HOMEDEFS.HOM file:
# Initialize the 8255 CIO/DIO board
start init_8255
{
outport 0203 9b # program the 8255 Control Register
}
end init_8255
Replace 0203 with the base address+3 of your 8255 board,
and replace '9b' with the Control Register value appropriate
for your setup, as it defines which ports are input vs. output.
See the table below "Control Register - "Mode 0" Operation"
for a list of all the possible I/O configuration control
register values.
2. Create an entry in either the AUTOEXEC.BAT file, or the batch
file you use to start OPCS, using:
home init_8255
..which programs the 8255's control register based on the
above HOMEDEFS.HOM file entry.
Note that individual bits can NOT BE programmed separately;
> Port A can be programmed to either be "all in" or "all out"
> Same for Port B
> Port C, the LOWER and UPPER 4 bits can be controlled
separately for Input or Output.
I strongly suggest referring to the 8255 and/or 82C55 data sheets
for authoritative information about how to program the 8255 for
I/O. Secondarily the manual for the CIO-DIO48 ISA card.
What follows below is my reduction of those docs. (erco@seriss.com)
Control Register - "Mode 0" Operation
-------------------------------------
This document ONLY covers "Mode 0" of the 8255, which is simple
"real time I/O" for all ports. Mode 1 (Strobed I/O) and Mode 2
(Bi-Directional Bus) are not covered here. For more info on those
features, refer to the 8255 data sheet.
Group A Group B
I/O Ctrl I/O Ctrl
______ _____
/ \ / \
_______________________________________________________________________
| | | | GROUP | | GROUP | GROUP A | GROUP B | |
|MS |M3 |M2 | A |M1 | B |_________________|_________________| |
|___|___|___|_______|___|_______| | | | |Ctl|
|D7 |D6 |D5 |D4 |D3 |D2 |D1 |D0 |PORT A|PORT C(HI)|PORT B|PORT C(LO)|Reg|
|---|---|---|---|---|---|---|---|------|----------|------|----------|---|
| 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | OUT | OUT | OUT | OUT |80 |
| 1 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | OUT | OUT | OUT | In |81 |
| 1 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | OUT | OUT | In | OUT |82 |
| 1 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | OUT | OUT | In | In |83 |
| 1 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | OUT | In | OUT | OUT |88 |
| 1 | 0 | 0 | 0 | 1 | 0 | 0 | 1 | OUT | In | OUT | In |89 |
| 1 | 0 | 0 | 0 | 1 | 0 | 1 | 0 | OUT | In | In | OUT |8A |
| 1 | 0 | 0 | 0 | 1 | 0 | 1 | 1 | OUT | In | In | In |8B |
| 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | In | OUT | OUT | OUT |90 |
| 1 | 0 | 0 | 1 | 0 | 0 | 0 | 1 | In | OUT | OUT | In |91 |
| 1 | 0 | 0 | 1 | 0 | 0 | 1 | 0 | In | OUT | In | OUT |92 |
| 1 | 0 | 0 | 1 | 0 | 0 | 1 | 1 | In | OUT | In | In |93 |
| 1 | 0 | 0 | 1 | 1 | 0 | 0 | 0 | In | In | OUT | OUT |98 |
| 1 | 0 | 0 | 1 | 1 | 0 | 0 | 1 | In | In | OUT | In |99 |
| 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | In | In | In | OUT |9A |
| 1 | 0 | 0 | 1 | 1 | 0 | 1 | 1 | In | In | In | In |9B |
-----------------------------------------------------------------------
/|\ /|\ /|\ /|\
| | | |
| | | All zeroes (for "Mode 0")
| | All zeroes (for "Mode 0")
| All zeroes (for "Mode 0")
All ones (for "Mode 0")
MS = Mode Set bit (hi bit of control register), must be 1 for "Mode 0"
M3,M2,M1 = must all be 0 for "Mode 0".
Here are all the possible 8 bit Control Register values (in HEX) to
program Ports A,B,C in "Mode 0" for all possible Input/Output combinations:
Control Register (HI 4) (LO 4)
Value (hex) PORT A PORT C PORT B PORT C
---------------- ------- ------- ------- -------
80 Out Out Out Out <- Port A,B,C=Out
81 Out Out Out In
82 Out Out In Out
83 Out Out In In
88 Out In Out Out
89 Out In Out In
8A Out In In Out
8B Out In In In
90 In Out Out Out
91 In Out Out In
92 In Out In Out <- Port A,B=In, C=Out
93 In Out In In
98 In In Out Out
99 In In Out In
9A In In In Out
9B In In In In <- Port A,B,C=In
(The above table is from the CIO-DIO48 ISA board manual for the 82C55)
Put simply, the bit mask for the control register I/O control for Mode 0
for ports A,B,C is:
Data Bit Mask Description
-------- ---- -----------
D0 01h Port C(LO)
D1 02h Port B
D3 08h Port C(HI)
D4 10h Port A
NOTE: "D2" bit is skipped because it's the M1 mode bit
So if the base address is 0310, one can program ports A,B and C
of the first 24 bits of I/O to all be inputs using the C code:
outp(0x0313, 0x9b); // Program ports A,B,C to be inputs
And to read the input bits for ports A,B,C:
int a_bits = inp(0x0310); // Read PORT A bits
int b_bits = inp(0x0311); // Read PORT B bits
int c_bits = inp(0x0312); // Read PORT C bits
The same can be done for the second 24 bits of I/O, by simply adding 4
to all the addresses above, e.g.
outp(0x0317, 0x9b); // Program 8255#2 ports A,B,C to be inputs
int a_bits = inp(0x0314); // Read second 8255 PORT A bits
int b_bits = inp(0x0315); // Read second 8255 PORT B bits
int c_bits = inp(0x0316); // Read second 8255 PORT C bits
Similarly, if ports are programmed to be outputs, you can WRITE bits
to ports A,B,C using outp() instead of inp().
NOTE: According to the 8255 data sheet, the Control Register can ONLY be
written to, and not read! In practice I've noticed 82C55 chips
let you read the Control Register and see the last value set.
The 8255.EXE program by default reads the Control Register
to see how to present the I/O data, unless the user specified
the Control Register value byte as the optional argument
after the base address, e.g.
8255.EXE 0310 9b
-------- ---- --
| | |
| | Control Register value optional argument
| |
| Base address optional argument
|
Runs the 8255 program, usually in
the \OPCS\BIN directory.
SEE ALSO
CIODIO24(DOCS) - Docs on the 8255 based 24 channel Digital I/O card
CIODIO48(DOCS) - Docs on the 8255 based 48 channel Digital I/O card
ORIGIN
Gregory Ercolano, Topanga, California 04/12/00